This invention relates to surface wave devices and more particularly to surface wave devices used in frequency discriminators. The general characteristics of surface wave devices are well known. A surface wave is launched by an input transducer disposed on a piezoelectric substrate. The surface wave energizes one or more output transducers also disposed on the piezoelectric substrate. The frequency response of the surface wave device is normally characterized by the formula f(X)=(sin X/X).sup.2 where X = N.pi. (f-f.sub.O)/f.sub.O in which N is a number of pairs of fingers in the interdigital transducer and f.sub.O is the synchronous frequency or the frequency at which the maximum response occurs. In a frequency discriminator two output transducers conveniently positioned on opposite sides of the input transducer can be used. One output transducer has a peak frequency response at a frequency slightly higher than the design center frequency of the input transducer, while the other output transducer has a peak frequency response at a frequency slightly lower than the design center frequency of the input transducer. Assuming proper phasing, the composite frequency response is a typical s-shaped frequency discriminator characteristic.
The (sin X/X).sup.2 response, however, gives rise to side lobes which in turn gives rise to spurious responses at various other frequencies. Furthermore, the idealized (sin X/X).sup.2 response does not account for various parasitic effects such as bulk wave coupling in the piezoelectric substrate, inductive and capacitive feed-through from the input to output transducers, and other similar parasitic effects. In various applications of a surface wave frequency discriminator such parasitic effects provide spurious responses which deleteriously effect the operation of the system. For example, in an automatic frequency control system the system may lock-in to a false or spurious output signal due to the spurious parasitic and side lobe response combinations.